There are several application areas that have a constant need for more efficient, more powerful, and better performing optical sources. Examples of such application areas include industrial processing, medical instrumentation, telecommunications, emerging high-definition compact disc recording devices, Light Detection and Ranging/Laser Detection and Ranging (LIDAR/LADAR) applications, and automotive applications. For several decades, research and development groups around the world have attempted to replace all types of lasers involved in these applications with more compact and efficient sources based on the direct use of semiconductor lasers.
An example of a laser is the laser diode. Laser diodes are ideal optical sources for many applications. They are efficient, compact, and readily available. Some applications for laser diodes have been extremely successful, such as in optical telecommunications and optical compact disc/digital versatile disc (cd/dvd) reader/writers. However, even though they are capable of high power output, their use as a direct light source have been primarily limited to applications that require only relatively low power. Their use as high-power direct light sources is limited by a complex mode structure of output emission at those power levels, preventing the simple and efficient coupling of the emission into a diffraction-limited beam.
To create high-power coherent beams, the light from the laser diodes is used as an energy pump for solid state lasers, such as the diode-laser-pumped solid-state lasers (DLPSS) and Raman/fiber lasers. This intermediate technical solution can be considered as an indirect (non-linear) mode reshaping. Such indirect techniques can produce significant power. However, they have significant fundamental conversion losses, and the total plug-in efficiency of such indirect sources is typically less than 30%. Furthermore, no direct-current modulation is possible with the laser-diode-pumped sources. These major drawbacks of indirect techniques have maintained the interest in direct semiconductor laser sources.